This invention relates to a gap measuring system for adjusting a gap or spacing between a transparent object which is capable of reflecting a light beam at least to a degree and a transparent or non-transparent object which is capable of reflecting a light beam at least to a degree.
In X-ray exposure apparatuses or proximity exposure apparatuses using ultraviolet rays, which are examples of pattern printing apparatuses, it is necessary that the spacing between a mask and a wafer be accurately adjusted so that a predetermined minute gap is maintained therebetween, and that the mask and wafer are held in a sufficiently parallel relation.
This has been conventionally achieved by mechanical adjustment using a dummy mask. The dummy mask has a projected portion having a height corresponding to the necessary spacing or gap. During adjustment, the dummy mask is used in place of an actual or real mask and the wafer is fixedly held with its photoresist surface directly contacted to the dummy mask so that the predetermined gap is established between the wafer and the actual mask position. This mechanical adjustment is, however, disadvantageous since the photoresist surface is liable to be damaged, which leads to decreased yield of the semiconductor devices. Further, the photoresist material adhering to the surface of the dummy mask causes errors in the parallelism setting and the gap setting. These problems can be overcome if the mask and the photoresist surface of the wafer are opposed to each other without any contact and the alignment is effected while maintaining such non-contact state.
A system for adjusting the gap between the mask and wafer using a plurality of white light interferometers has already been proposed by the same assignee of the subject application in a Japanese Patent Application Laid-Open No. 52-52579. Also a white light interferometer using a Wollaston prism has already been proposed by the same assignee of the subject application in a Japanese Patent Application Laid-Open No. 52-4260.
FIG. 1 shows such white light interferometer using the Wollaston prism, and this is incorporated into the embodiments of the present invention which will be described later. In FIG. 1, designated by a reference numeral 1 is a light source such as a tungsten lamp providing a light beam 2 having a wavelength range. The light beam 2 illuminates an object 3 to be measured (e.g. a plastic film or air spacing between the mask and wafer) having an index of refraction n and a thickness d. A portion of the light beam 2 is reflected by a first surface 31 of the object 3 so that it is deflected as a light beam 5. Another portion of the light beam 2 is refracted by the first surface 31 of the object 3 and then reflected by a second surface 32 of the object 3 so that it is deflected as a light beam 4. The light beam 4 is again refracted by the first surface 31 so that it is deflected as a light beam 6. For the sake of simplicity of description, a single wave surface or wavefront 51 at one moment is selected as a representative of the light beam 5, while a single wavefront 52 at the same moment is selected as a representative of the light beam 6. As is well known in the art, the optical path length difference between the wavefront 51 and the wavefront 52 is expressed as 2.multidot.n.multidot.d cos.phi., where .phi. is the angle of incidence on the second surface 32 of the object 3.
The arrangement shown in FIG. 1 includes an interference fringe forming portion 50 which is disposed to receive the wavefront 51 and the wavefront 52. The interference fringe forming portion includes a polarizer 53, a Wollaston prism 60 and an analyzer 56 which are disposed in this order along the optical axis. The Wollaston prism 60 consists of a combination of a prism 54 made of a birefringent or double refracting crystal material, such as a crystal or calcite, which has been cut so that the optic axis thereof extends in a direction perpendicular to the plane of the sheet of the drawing, with a prism 55 made of a similar birefringent crystal material which has been cut so that the optic axis thereof extends longitudinally as viewed in the drawing. The polarizer 53 is disposed so that its optic axis defines an angle of 45 degrees relative to each of the optic axes of the crystals 54 and 55. The analyzer 56 is disposed relative to the polarizer 53 so that they provide parallel or crossed Nicols.
In this arrangement, the polarizer 53 provides a linearly polarized light which vibrates in the direction having an angle of 45 degrees relative to the plane of the sheet of drawing. This linearly polarized light enters into the prism 54 whereby it is separated into ordinary rays and extraordinary rays. These ordinary rays and extraordinary rays are in turn incident on the prism 55 in which the ordinary rays advance as extraordinary rays while, on the other hand, the extraordinary rays advance as ordinary rays. Because of the difference in the refractive index of the birefringent crystal material with respect to the ordinary rays and extraordinary rays, the rays emitted from the prism define wavefronts which are inclined relative to each other. As the result, the wavefronts 51 and 52 are transformed into wavefronts 51', 51" and wavefronts 52', 52", respectively, which produce interference fringes having intensity distributions under action of the analyzer 56. The optical path difference between these wavefronts can be expressed as 2(n.sub.e -n.sub.o).multidot.Y.multidot.tan.theta., where n.sub.o is the refractive index of the birefringent material with respect to the ordinary rays, n.sub.e is the refractive index of the birefringent material with respect to the extra-ordinary rays, Y is the co-ordinate with respect to an origin which is coincident with a point at which two prisms have the same thickness and measured in the direction orthogonal to the optical axis, and .theta. is the optical angle of the prism.
The wavefronts 51' and 51" interfere with each other while the wavefronts 52' and 52" interfere with each other, so that in the neighborhood of points 58 (Y=0) shown in FIG. 1, peaks of the white light fringes are formed respectively.
On the other hand, the wavefront 51' and the wavefront 52" interfere with each other in the neighborhood of a point 57 shown in FIG. 1, while the wavefront 52' and the wavefront 51" interfere with each other in the neighborhood of a point 59, so that side peaks are formed thereat, respectively. The points at which these peaks are produced are such points at which: EQU 2.multidot.n.multidot.d cos.phi.=2(n.sub.e -n.sub.o).multidot.Y.multidot.tan.theta..
At each of these points, an optical path length difference of such amount which just cancels the optical path length difference caused by the object 3 is produced by the Wollaston prism P. From the above-described relation, the side peaks are formed in the neighborhood of: EQU Y=.+-.n.multidot.d.multidot.cos.phi./{(n.sub.e -n.sub.o)tan.theta.}.
These white light fringes are projected by a lens 69 onto a photodetector 30.
The white light fringes projected on the photodetector 30 are schematically illustrated in FIG. 2. In this Figure, a reference character Fc designates a center interference peak while reference characters Fs and Fs' designates side peaks, respectively. When the Wollaston prism has a given optical angle .theta. and the object to be measured has a refractive index, measurement of the distance between the peaks Fc and Fs or the distance between the peaks Fc and Fs' serves as a measure of the thickness of the object to be measured or the distance between the surfaces of the object to be measured, since the distance between the peaks as aforesaid is proportional to the thickness of the object to be measured or the distance between the surfaces of the object.
FIG. 3 shows an arrangement for measuring and adjusting the gap between a mask and a wafer with the use of white light interference fringes, such as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 52-52579. In FIG. 3, elements corresponding to those shown in FIG. 1 are denoted by the same reference numerals. The light beam emitted from a white light source 1 is condensed by condenser lenses 23 and 24 and then is reflected by a half mirror 26 to an objective lens 27. By this objective lens 27, the light beam is collimated so that a parallel beam illuminates a mask 11 and a surface 201 of a photoresist layer 14 formed on a wafer 12. Designated by a reference numeral 25 is a color filter inserted into the path of the illumination light in order to prevent sensitization of the photoresist 14. The filter 25 may usually be a yellow filter absorbing the rays having wavelengths not greater than 500 nm. If the optical system is arranged so that the transparent portions of the mask 11 are illuminated by the illuminating light beam, the rays reflected by the mask 11 surface and the photo-resist surface 201 are converged again by the objective lens 27 and, after transmitted through the half mirror 26, are collimated by another collimator lens 27'. The combination of the visible light with the transparent portions of the mask 11 may of course be replaced by a combination of X-rays, which is invisible, with a mask therefor. This is because the mask to be used in the X-ray exposure is very thin, so that it can be considered in essence as being approximately transparent relative to the longer-wavelength region of the visible range and to the infrared range.
In the path of the advancing parallel light beam formed by the collimator lens 27', there is provided an interference fringe forming portion 50 comprising a polarizer 53, a Wollaston prism 60 and an analyzer 56. With this arrangement, white light interference fringes having information on the distance or gap between the mask and wafer with respect to the area of the photomask which is being currently illuminated are formed on the Wollaston prism 60. The image of interference fringe is projected through a projection lens 69 on a photodiode array or image pickup tube 30, and, on the other hand, is reflected by a half mirror 28 so that it is observed by an observer 14 through eyepiece lenses 31 and 32. Automatic adjustment of the gap between the mask and wafer is performed in the following manner. First, the distance between the center peak Fc and the side peak Fs detected by a photodetector 30 such as a photodiode array is compared with a reference value in a signal processing circuit 36. On the basis of the results of comparison, an actuator control circuit 37 drives one or more of actuators 35, 35' and 35" in the directions as denoted by double headed arrows shown in FIG. 3.
Subsequently, by moving the mask relative to the measuring optical system or by using a plurality of measuring optical systems, the gaps between the mask and wafer at a plurality of points are measured. The signal processing circuit 36 compares the results of measurement with the reference value. On the basis of the results of comparison, one or more of the actuator rods 35, 35' and 35" are driven in the directions as denoted by the double-headed arrows through the actuator control circuit 37, whereby a constant and predetermined distance or gap is achieved between the mask and the wafer at the plural points.
If a thin layer such as the photoresist layer 14 exists in addition to the air space, as shown in FIG. 3, the gap measurement can be achieved without any specific difficulties. This is explained with reference to FIG. 4A and 4B. The FIG. 4A shows the white light fringes in a case where an object having a thickness d.sub.1 and a refractive index n.sub.1 is measured. The abscissa shows the space co-ordinate while the ordinate shows the intensity of light. FIG. 4B shows the fringes in a case where a material having a thickness d.sub.2 and a refractive index n.sub.2 is formed on an object having a thickness d.sub.1 and a refractive index n.sub.1. In this case, the value of n.sub.2 .times.d.sub.2 is not greater than 0.5 microns which is small as compared with the value n.sub.1 .times.d.sub.1. Usually, the thickness of the photoresist used in the patterning of the semiconductor device is not greater than 0.2 microns, while the air gap used in the X-ray exposure or proximity exposure process is in a range of 5-10 microns, which is not less than 20 times larger than the thickness of the photoresist.
Thus, in this case, the form of the center peak can be regarded as being slightly expanded, such that the measurement of the air gap can be achieved without any specific inconveniences.
The above-described gap adjusting method may be satisfactory if each of the mask and the wafer has a relatively small diameter and has sufficient flatness. If, however, the diameter of the wafer is enlarged, several problems arise therefrom.
For example, when a larger diameter wafer such as a 6-inch wafer or a 7-inch wafer is to be used as compared with the conventional 3-inch wafer, it is difficult to maintain the flatness without using any specific flatness correcting means. In the X-ray exposure system, for example, an air gap of 10.+-.1 microns must be maintained between the entire mask and wafer. When, however, the wafer diameter is not less than 6 inches and if the wafer has been subjected to various semiconductor circuit manufacturing processes, the wafer surface will include complicated irregularities. These irregularities can not be sufficiently removed by the actuators 35, 35' and 35" only. In order to correct such irregularities to assure practically sufficient flatness, it has been proposed and actually practiced that a holder for holding the wafer by vacuum suction is divided into plural elements and a load is applied to each of the respective actuators to achieve flatness correction. This method however does not assure very satisfactory results unless the gap between the mask and wafer at multiple points on the wafer is measured substantially at the same time. Such multi-point measurement will be achieved, for example, by enlarging the optical system of the gap measuring device so that the entire surface of the wafer can be observed at the same time. However, this is inconvenient for the following reasons:
First, if the optical system shown in FIG. 3 is enlarged without any significant change thereto, the optical system becomes too bulky and weighty.
Second, the manufacturing cost thereof becomes too high.
For example, when the gap measuring optical system is arranged so that the wafer and the Wollaston prism are disposed in a one-to-one imaging relation, a 5-inch wafer requires a 5-inch crystal plate while a 7-inch wafer requires a 7-inch crystal plate.